Report on the geologic and stratigraphic logging project of Semiliki Basin-Albertine Graben in Ntoroko District Western Uganda
Mutebi , Henry
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The geologic and stratigraphic logging project was conducted in semliki basin, Ntoroko district, western Uganda. Semliki basin is located in the central domain of the western arm of the Albertine graben. It is a pull apart basin formed when the Albertine graben, a product of active rifting, underwent transtensional strike-slip deformation that was controlled by already existing NE-SW graben-forming normal faults. The main aim of the field study was to acquire field training on how to collect and interpret stratigraphic, petrographic, sedimentologic and structural data which are important in 3hydrocarbon exploration. Some of the materials used during the study included a geologic hammer, Grain Size Scale, Jacob staff, handheld GPS, notebook and a geologic compass. The methods involved desk Study conducted at camp, fieldwork, sedimentary logging exercise, data collection, discussions, analysis and interpretation. The Semliki basin is covered by sediments that represent Middle Miocene to recent exhibiting a fining upward sequence underlain by possible Jurassic to Early Tertiary age sediments resting unconformably over the basement rocks. The stratigraphic sequence exhibited in the basin is divided into seven Formations (in decreasing age) namely; the Kisegi, Kasande, Kakara, Oluka, Nyaburogo, Nyakabingo and Nyabusosi formations. Four lithofacies were identified indicating different depositional environments (fluvial, deltaic and lacustrine). Lithofacies 1 is poorly sorted and coarse-grained sediments consisting cobble/pebble-size cemented together with fine to coarse grained sands and formed the contact between the basement and sediments. Lithofacies 2 is a massive reddish brown, coarse to medium quartz sands. Lithofacies 3 is grey, dark brown and army green clays with medium quartz sands and plant debris. Lithofacies 4 is grey to white silt. The basin is affected by intensive tectonic activity, which led to the formation of vast types of structures that include faults, joints, bedding, cross-bedding, laminations, unconformities, mud diapirs, and plunging folds among others. Geophysical data of semliki basin and shallow, high resolution (150-200 Hz) seismic data from an intraslope salt withdrawal minibasin in the Gulf of Mexico were availed for interpretation. This data was later analysed by softwares such as Oasis montaj (Geosoft) and sedilog,.The sediments of the Semliki Basin represent a petroleum play for hydrocarbon accumulations as all elements of a petroleum system were identified in the field. The presence of oil seeps in kibuku indicates that organic rich source rocks are present, an indicator of an active system thus great petroleum potential. Information from the field visit to sempaya hot spring indicate the presence of high enough geothermal gradients of over 67°C/km for the maturation and generation of hydrocarbons at shallow depths.